A variety of substances have previously been proposed as organic photoconductive materials to be used in electrophotography, such as, for example, N-vinylcarbazole and its derivatives. However, very few colored or color developing organic photoconductive materials are known which are obtained by coupling organic photodonductive materials with coloring materials or color developing components.
The present inventors, who have investigated the reproduction of multi-colored or naturally colored images as one of the aims in electrophotography, obtained good results when they used colored organic photoconductive materials as toner in the production of superimposed images. For example, in the reproduction of naturally colored images on a panchromatically sensitized zinc oxide photoconductive layer, a negatively charged photoconductive layer is first exposed to the object through a red filter and then developed with a cyan-colored toner. The photoconductive layer is then again charged in a negative sense and exposed to the object through a green filter to obtain a negatively charged image which is subsequently developed with a magenta toner. Further, the photoconductive layer is charged negatively, exposed to the object through a blue filter and developed with a yellow toner.
In this case, if the images toned in cyan color are electrically insulating, they are also charged at the second negative charging and the charged electricity will not disappear at the following exposure to green light so that the images produced by the magenta toner inevitably overlap the images in cyan. On the other hand, however, if the cyan-colored toner is electrically conducting, the cyan images are not at all charged during the second charging process so that, if the images produced by the second exposure with the green light overlap the blue images of the first exposure, the magenta toner will never be attached to the respective part. To overcome the difficulty, the toner has to exhibit photosensitivity in itself.
For obtaining satisfactory results it is, however, not sufficient that the toner possesses photosensitivity or photoconductivity. If the magenta toner and the yellow toner are not transparent to light, for example, the parts where the images of three colors superimpose each other will not show their mixed color (in other words, will not show blue color at the superimposition of cyan and magenta, nor green color at the superimposition of cyan and yellow), but the color of the toner employed afterwards covers the color of the previously applied toner, so that the parts where cyan is superimposed by magenta and by yellow show magenta and yellow color, respectively.
In conclusion, the toners, in either dry or wet process of developing, must be of colored materials having photoconductivity and light transparency to reproduce naturally colored images by electrophotography.
Organic photoconductive materials colored with a coloring material and meeting the above requirements have previously been investigated. But the lot-to-lot variation of the coloring materials with respect to purity, solubility etc. often resulted in fluctuation in the quality of product, and therefore reproducible and continuous production of the developers was very difficult. It was also found that the coloring materials in the developed images became blurred during storage and have a tendency to transfer to other matter.
The present inventors continued their efforts to eliminate these defects and finally arrived at the present invention.